Photoemitter structure including porous layer of photoemissive material

ABSTRACT

A photoemitter of improved quantum efficiency is formed by smoke or low density deposition of photoemissive materials on a substrate. Significantly, the photoemissive layer is deposited in the presence of a low pressure gas to form a layer whose density is not greater than 20% and preferably not greater than 5% of the photoemissive material in its bulk form. Individual particle size is controlled and deposited particles are isolated, affording greater surface contact area with the subsequently deposited constituent materials of the photoemissive surface, thereby enhancing interaction and increasing the photoelectron emission. The low density, randomly oriented smoke deposits provide a photosensitive surface characterized by high absorption, low reflective losses, and low transmissive losses. The spectral response curve of smoke photoemitters peaks further into the red or near infrared region than prior art devices, rendering the smoke photoemitters of the invention ideally suited for use as near infrared sensors in low light level imaging system.

United States Patent 191.

Bo'nrg, Jr. et a1.

-. my 3,309,945 1 11 May 7, 19174 Related US. Application Data [6-3] Continuation-impart of Ser. No. 110,057, Jan. 27,

1971, abandoned.

[52] US. Cl. 313/94, 313/215 [51] Int. CLLL HOli 39/00 [58] Field of Search 3137215, 94; 117/219 [56] References Cited l UNITED STATES PATENTS 2,120,916 6/1938 Bitner ..ll7/2l9 REFLECTIVE LDSSES INCIRENT RADI now: 5 SHORTER p; wnyrl ll ii m e PHOTONS ABSORBED 4! V SUBSTRATE Primary Examiner-Archie Borchelt Q Assistant Examiner-Davis L. Willis Attorney, Agent, or FirmC. L.-O"Rourke 571 2 ABSTRACT A photoemitter of improved quantum efficiency is formed by smoke or low density deposition of photoemissive materials on a substrate. Significantly, the photoemissive layer is deposited in the presence of a low pressure gas to form a layer whose density is not greater than 20% and preferably not greater than 5% of the photoemissive material in its bulk form. Individual particle size is controlled and deposited particles are isolated, affording greater surface contact area with the subsequently deposited constituent materials of the photoemissive surface, thereby enhancing interaction and increasing the photoelectron emission. The lovv density, randomly oriented smoke deposits provide a photosensitive surface characterized by high'absorpt'ion, low reflective losses, and low transmissive losses. The-spectral response curve of smoke photoemitters peaks further into the red or nearinfraredregion'than prior art devices, rendering the smoke pho-. toemitters of the invention ideally suited for use as near infrared sensors in low light level imaging system.

I 8 Claims, 5 Drawing Figures HARD VACUUM DEPOSlTED PHOTOEMITTER PHOTO ELECTRONS 9 J i PHOTON IS TRANSMITTER 'FRANSMISSION tossEs T'ILM THICKNESS=50nm EVACUATE CHAMBER INTRODUCE PRESSURE F 16.! INERT GAs CONTROL PRESSURE FORM BICK SMOKE CONTROL FIRST CONSTITUENT HEAT TO FORM vAPoR OF FIRST EvAGuATE 0nd ESTABLISH IN CHAMBER I HARD VACUUM CHAMBER TEMPERATURE OEPOsITION OF CONTROL REMAINING CONSTITUENTS SENSE RESPONSE HEAT TO FORM cIIARAcTERIsTIcs vAPoR OF 0F DEVELOPING SELECTIVE CONTROL REMAINING PHOTOSURFACE 1 c N IT OF RATE OF 0 ST I l INTRoOucTION 0F J REMAINING CONSTITUENTS PHOTOEMITTER STRUCTURE INCLUDING POROUS LAYER OF PHOTOEMISSIVE MATERIAL This application is a continuation-in-part of application Ser. No. 110,057, filed Jan. 27, 1971 and now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention I This invention relates to photoemitters and, more particularly, to photoemitters of improved quantum efficiency formed by smoke or low density deposition of photoemissive materials.

2. State of the Prior Art Photosensitive surfaces, and particularly photoemitters, have long been known in the prior art. Numerous techniques have been attempted heretofore to increase the quantum efficiency of such surfaces. Many of these efforts have been directed to changing the processing schedule employed in-the formation of conventional surfaces, as well as the investigation of new photoemissive materials having higher quantum efficiencies. Generally, as to the prior art techniques which have been successful in increasing the quantum efficiency, there results a shift in the spectral response curve of the surface to the ultraviolet and away from the infrared and near infrared regions of the spectrum.

Another prior art technique has been to increase the thickness of the surface in an effort to increase the absorption; this, however, typically resultsin increasing the reflectance of the surface, rather than increasing the absorption, with a concomitant reduction in effi-' ciency.

Photoemitters are formed both of metallic and nonmetallic or semiconducting materials and, in some instances, combinations thereof. The efficiency of semiconducting photoemitters is greater by a factor of or more than that of metallic photoemitters produced under the identical processes. When considered simply as optical to electrical energy conversion elements, semiconducting photoemitters are far more efficient than metallic photoemitters, for photon energies above the band gap of the material. However, semiconducting photoemitters donot demonstrate satisfactory quantum efficiencies for the longer wavelengths of light, and particularly the infrared and near infrared regions.

Low light level imaging systems have been developed which place exacting requirements on photosensors employed therein, requiring a reasonably high quantum efficiency in the near infrared regions for producing a usable response. For example, active imaging systems utilizing gallium arsenide and neodymium doped lasers require detectors having high quantum efficiency at 850 nanometers and 1.06 micrometers, respectively.

Prior art efforts to produce suitable photoemissive surfaces with these exacting characteristics have resulted in, the development of improved semiconducting photoemitters; nevertheless, none of the presently developed semiconducting photoemitters has characteristics which are any better than the conventional, socalled S-1 photoemitter for practical applications in use for detecting radiation in the one micrometer wavelength range.

Although metallic photoemitters have been known for several decades, the emission mechanism is still not understood completely. By contrast, solid state physics has greatly added to the understanding of the emission mechanism in semiconducting photoemissive materials. Some photoemitters have both metallic and semiconducting characteristics, and the emission mechanism of these is at best vaguely understood. The S-l surface, which is formed of silver, oxygen, and cesium, is an example of the latter.

lt is instructive nevertheless to review the procedures involved in photoemission, at least to the extent they are presently theoretically understood. The procedure may be divided conveniently into three process steps. The first step is that of photon absorption, i.e., the absorption of energy from the incident radiation. Absorption of photons creates so-called fhot electrons, i.e., electrons having energies exceeding the energy levels of electrons which are existing in the solid in thermal equilibrium. The photon absorption process is more efficient in semiconducting photoemitters by a factor of 10 or better than in metallic photoemitters produced under the identical processes, due in large part to the high reflective characteristics of the metals employed. This contributes greatly to the much higher quantum efficiencies of the semiconducting photoemitters.

In the second step, the hot electron moves to the interface between the photoemissive surface and the vacuum environment. in which it is used. This motion results in energy loss of the bot electron. In metallic emitters, the energy loss process results from electron scattering, whereas in semiconducting materials it results from photon scattering. Electron scattering introduces greater losses than photon scattering, and thus it is much more difficult for a hot electron generated below the surface of the metallic material to escape as a free photoelectron. Thus, whereas semiconducting photoemitters have escape depths of several hundred angstroms, the escape depths in metallic emitters are much smaller.

Therefore, the higherthe light absorption coefficient of the material, in accordance with the first step, the greater the number of hot electrons produced and, in accordance with the second step, the greater the probability of photoelectron escape.

As a final basicstep in the photoemission process, the hot electron must escape from the material'into the vacuum. This escape requires that the hot electron overcome the surface barrier energy level of the photoemissive material. In metallic photoemitters, the energy necessary to overcome the surface barrier is determined by the work function of the material whereas in semiconductors it is determined by the electron affinity of the material, i.e., the difference in energy between the vacuum level of the system in which the semiconducting photoemitters .is employed and the lowest conduction band of the material. Again, greater energy is required for escape from the metallic photoemissive material, further contributing to a reduced quantum efficiency.

In most respects, therefore, the quantum efficiencyv of semi-conducting compounds found to be useful as photoemissive materials is higher than metallic compounds. Nevertheless, the threshold of photoemission for semiconducting materials does not extend as far into the near infrared region as has been experienced with certain metallic photoemitters; particularly, the

S1, or silver-oxygen-cesium phot-osurface has been recognized to be superior to any other surface for this purpose. For example, this surface has a quantum efficiency of 0.1% even as far into the infrared region as 1 micrometer, although its peak quantum efficiency throughout the visible wavelength range is much lower than most other surfaces.

Further, although the photoemissive characteristics of many new semiconductingcompounds have been investigated, such as binary and ternary compounds of Groups III-V of the Periodic Table, the'technology of producing such surfaces is not well developed and response levels appear to be limited by present state-ofthe-art techniques for producing clean surfaces in contamination-free processing chambers. Utilization of such devices with light transmitting substrates wherein the photosurfaces are irradiated from the substrate interface rather than the vacuum interface is still beyond the present state of the art. Other efforts, such as varying the relative proportions of the constituents of the photoemitting compounds, superficial oxidation, and field assisted photoemission have failed to provide photoemissive surfaces having adequate response characteristics in the near infrared region.

The present invention overcomes these and other problems experienced with, and deficiencies in, prior art photoemitters and techniques for their'production. Particularly, the invention includes both a method of production and resultant product comprising a new type of photosurface having higher quantum efficiency, while nevertheless utilizing known photoemissive materials. Further, the smoke photoemitters of the invention afford an improvement in the quantum efficiency in the near infrared region of the spectrum satisfying the present need for image transducers having such characteristics. In addition, the method of the invention may be practiced in, and the product of the invention realized from, the use of presently available processing techniques and equipment.

SUMMARY OF THE INVENTION The invention comprises the use of smoke deposition in the processing steps for producing a photoemitter and also the novel structure of the resultant photoemitter. More precisely, the initial deposition on a substrate of a constituent of the photoemissive material is per formed by smoke deposition in a low pressure inert gaseous atmosphere, followed by the further deposition of the remaining constituents of the photosurface.

The resulting photoemitter comprises a low density, randomly oriented chain of discrete particles or smoke deposits affording greatly increased surface contact area with the remaining constituents which are deposited thereover in a conventional high vacuum deposition procedure. In particular, the photoemitter layer is a porous spongy element whose density is not greater than 20% and preferably not greater than 5% of its constituent material in bulk form.

By controlling the pressure of the inert gaseous atmosphere employed during the smoke deposition, control As currently understood, the efficiency of a photosurface may be improved by controlling the particle size produced within the photoemissive layer; thus, by con.- trolling the pressure of the inert gas within the chamber, the characteristics of the photosurface produced in accordance with the method of the invention may be improved. Likewise in accordance with the current understanding of the photoemissive phenomenon, such as the theory of a tunneling process to explain the emission technique, the larger surface contact area afforded over the individual particle size and density is realized.

by the discrete smoke particles developed by the smoke deposition technique of the invention also affords greater efiiciency of the resultant photosurface.

Various other characteristics of the novel photosurfaces of the invention also offer an explanation for the greatly improved operating characteristics. Generally, the low density smoke deposit tends to prevent both reflective and transmissive losses; in addition, optimum thickness of the photosurface can be provided without producing high reflectance; further, the low density, random nature of the smoke deposit affords substantial internal reflection providing virtually complete absorption of light energy while nevertheless enhancing the probability of total escape of the photoelectrons.

The photosurfaces resulting from the practice of the invention generally demonstrate improved spectral response characteristics over those produced by conventional processing techniques. In one area of specific, albeit limited interest, i.e., that of response in the near infrared regions, use of the smoke deposition technique of the invention provides a photoemitter having a spectral response curve peaking further into the'near infrared region than devices heretofore available in the prior art. The smoke photoemitters of the invention thus are ideally suited for use as near infrared sensors in low light level imaging systems.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 comprises a brief flow diagram illustrating the procedure for manufacturing a smoke or low density photoemitter in accordance with the invention;

FIG. 2 is a view of a portion of a photoemitter manufactured in accordance with the invention;

FIGS. 3a and 3b are diagrammatic illustrations representing the structure and functional characteristics of a hard vacuum deposited photosurface of the prior art and of a smoke deposited photoemitter in accordance with the invention; and

FIG. 4 comprises a graph comparing the spectral response curve of an S-l smoke photoemitter, in accordance with the invention, with that of a conventional S-l photoemitter.

DETAILED DESCRIPTION OF THE INVENTION In accordance with the invention, it has been discovered that by changing a processing step in the production of photoemitters, a substantial improvement in the quantum efficiency of the resultant device may be realized. This improvement is obtained as a result of greatly increased absorption of incident light energy with substantially decreased transmissive and reflective losses; as further detailed hereinafter, numerous additional benefits including control of particle size, increase of surface contact area of the constituents of the photoemitter, an ability to provide an optimum thickness of the photoemitter, and others, are also realized.

The invention has general applicability to the production of improved photosurfaces, and is disclosed herein in relation to certain specific metallic, and semiconducting-metallic, photoemissive surfaces. Even as to various new, i.e., experimental, semiconducting photoemitters, the use of the method of the invention greatly enhances the photoemissive characteristics of the new or experimental, as well as well-known, semiconducting photoemissive materials.

Whereas general improvement in the quantum efficiency of all such surfaces may be realized by the practice of the invention, an additional and very significant benefit of the invention arises from the change in the spectral response characteristics of at least one specific photosurface of the invention. More precisely, the smoke deposited S-ll photosurface exhibits a peak in its spectral response curve which extends further into the near infrared region than heretofore realized with the materials and processes of the prior art.

More specifically, the invention comprises effecting a smoke or low density deposition of a first constituent of the surface on a substrate, in a first series of steps in forming the surface. There results a so-called smoke surface of randomly oriented, discrete metallic particles connected together in a link configuration and presenting substantial voids therebetween. For example, the metallic material may comprise silver which is evaporated onto the substrate within a chamber containing an inert gas at a controlled pressure. The substrate maybe of glass or other suitable material.

The smoke deposits are then b'aked during conventional hard vacuum deposition of the remaining constituents, for example, an alkali, such as cesium. The hardvacuum deposition of the alkali may comprise the conventional steps of effecting a' deposition of the evaporated cesium while oxygen is introduced into the chamber, the amounts of cesium and oxygen being controlled to achieve the desired response characteristics of the surface.

This procedure is generally outlined in the flow dia gram of FIG. 1. Assuming the chamber to have been decontaminated, and an appropriate substrate received therein, the chamber is exhausted and then an inert gas such as argon is introduced to bring the chamber to the appropriate pressure level.

[n the case of the silver-cesium-oxide surface, the metallic constituent, silver, is evaporated such as by conventional filament heating techniques within the deposition chamber. The metal vapor then deposits on the substrate.

Smoke deposits are, of course, well known to those skilled in the art; however, smoke depositions have never been used heretofore in the production of photoemitters. For the purpose ofthe present disclosure, two specific examples of the method steps in making smoke photoemitters in accordance with the invention are given. The invention, however, has general applicability to the manufacture of smoke photoemitters of various constituent materials, and the necessary adjustments in operating parameters, although not specified herein for these other, various constituent materials, will be apparent to those skilled in the art.

In general, the metallic constituent is heated to achieve an evaporation rate sufficient for achieving a black smoke. This function is controlled by varying the temperature. However, since the evaporation rate varies widely for different materials, the amount of heating required also varies; in addition, the evaporation rate is pressure-dependent and thus will vary with the inert gas pressure in the chamber. The vapor pressure of the metal, however, is primarily a function of temperature.

As examples of suitable materials used as the first constituent deposited as a black smoke, and metals representative of these variable characteristics, antimony has a relatively high vapor pressure and need not even be heated to its melting point to achieve a sufficient evaporation rate to form a black smoke. lllustratively,

the souce of antimony may be heated to a temperature in the orderof 750K to achieve a sufficient evaporation rate. Conversely, silver is very stable, wtih a low vapor pressure, and thus must be heated to its melting temperature of 1234K to develop an adequate black silver smoke. In the processing, in fact, it will be observed that a white silver smoke initially develops, and which becomes black upon establishing the proper operating parameters.

In the manufacture of an 8-1 surface, as one specific example, the inert gas pressure and the rate of evaporation of the silver are controlled so as to achieve a black smoke of the silver particleswithin the chamber, for deposition on the substrate. In accordance with one successful practice of the invention, the inert gas pressure was in the range of from 34mm Hg. The smoke deposition was performed at room temperature in the range of 20 to 30C (although perhaps slightly elevated in temperature due to the heating of the metal) and took from 15 to 60 seconds for developing a black smoke deposit in the range of from to 300 Angstroms in thickness over a substrate having a surface of 2 inches diameter. The individual smoke particles thus deposited ranged from 2 to 5 micrometers in width and 5 to 50 micrometers in length. The inert gas may be il lustratively argon or helium and the source of silver may be disposed a distance in the range of l to 8 inches from the substrate or support member upon which the smoke layer is to be deposited. The resultant layer of silver has a density in the range of 1% to 10% of silver in its bulk form. in one illustrative process where the inert gas is helium and the silver source is disposed a distance of 5 inches from the substrate, a silver layer having a density of approximately 2% of its bulk density will result.

Upon conclusion of the smoke deposit, the chamber was exhausted to establish a hard vacuum. The substrate with the deposited smoke particles was then heated in conventional fashion, for example to a temperature of C. Under this condition, the further constituents of the photoemissive material were evaporated into the evacuated chamber, and the substrate with the smoke deposit thereon was heated to assure the deposition of the further constituents, e.g., the cesium oxide, onto the smoke deposits in intimate relationship.

A conventional cesium generator was utilized in this operation and oxygen was metered into the chamber by conventional methods.

As typically done in effecting hard vacuum depositions in the formation of photoemitters, the characteristics of the surface were continuously monitored as the deposition proceeded. The chamber was operated under conventional parameters and the constituents of oxygen and'cesium vapor were released into the chamber, as required, to establish the desired operating characteristics of the surface.

More specifically, the chamber pressure was about 1 X 10 torr, the cesium vapor and oxygen were intro-- duced at a temperature of 150C over aperiod of about 10 to 30 minutes. The cesium oxide formed during the hard vacuum deposition deposited over the surface of the silver smoke-particles, entering the voids therebetween and thus generally surrounding the particles.

The smoke photoemitter surface of the deposited smoke silver particles tends to act as a neutral density filter. By contrast to conventional, hard vacuum metallic depositions wherein the metallic constituent presents a high reflectance, the neutral density filter resulting from the smoke deposit has substantially reduced reflectance and, in fact, increased absorption.

In FIG. 2 is shown an illustrative drawing of a portion of a photoemissive surface formed in accordance with the invention. As therein illustrated, the surface resembles semicontinuous links or chains of particles having substantial voids therebetween.

As noted above, the emission mechanism in photosurfaces is not entirely understood even as to semiconducting photoemitters and the art is even less knowledgeable about the mechanism in photoemitters having both metallic and semiconducting properties. The 8-1 or silver-'cesium-oxygen photosurface is an example of the latter type. The analysis of the improved results obtained by the practice of the invention even as to the specific S-l surface, however, serves to illustrate the general applicability of the procedure of the invention in making photoemitters of various constituent materials, including combinations of metallic and semiconducting constituents, as well as those of primarily semiconducting consituents.

For example, for very short wavelengths such as in the ultra-violet region, a thin film of silver is capable of absorbing photons or incident light energy and emitting photoelectrons. Semiconducting materials similarly have this property. It is contemplated that binary and ternary compounds of Groups III and V of the Periodic Table, i.e., compounds formed of one or more elements from each of these groups, may be deposited as a low density layer in accordance with teachings of this invention. In the longer wavelength region and particularly in the near infrared region, it appears that a silver film operates to absorb photons. and to generate hot electrons internally. Release of the hot electrons as photoelectrons, however, requires a further cooperating constituent, provided in the case of the 8-] surface by the cesium oxide, which comprises the semiconducting constituent.

One theory of explanation for the photoemissive mechanism is a type of tunneling process. It is clear that the smoke deposition technique of the invention, by providing a much greater surface contact area between the metallic and semiconducting constituents, greatly enhances the interaction therebetween in the combined process of steps of photon absorption and release of free photoelectrons. Thus, the greater surface contact area afforded by the smoke deposition of the invention provides improved operating characteristics for any photoemitter which employs such an interaction mechanism.

The improved results obtained by the practice of the invention may also be understood from a generally optical approach. Whereas conventional prior art photoemitters deposited in high vacuum, typically 10' to 10 torr, comprise a relatively-dense continuous thin film in the order of l00%, of its bulk density, losses are incurred both in transmission of incident radiation through the film as well as reflection from the interface at the film. These effects are illustrated in FIG. 3a. In addition, characteristics of good reflection are that a reflector have excellent and continuous contact with a substrate. These conditions also obtain in the prior art structure illustrated in FIG. 3a.

By contrast, a smoke photoemitter in accordance with the invention is shown in FIG. 3b. The black losses if for no other reason than the general opacity of the resulting layer. In addition, the minimal contact between the layer and the substrate provides very poor reflection of incident radiation at the interface. Thus, both transmissive and reflective losses are substantially reduced again, regardless of the specific constituents.

Ignoring reflection at the interface, incident light is subject to reflection by the constituents of the surface, whether prepared in accordance with the-prior art or the invention. In the surface of the invention, however, the angle of reflection varies from 0 to further due to the porous nature of the surface, incident light can penetrate more deeply into the layer before reflection takes place. When reflection does take place, a substantial probability exists that the reflected energy will strike and be absorbed by adjacent smoke particles. In fact, the only incident light which, at least theoretically, is capable of being reflected is that having an angle of incidence relative to the substrate of 90 and which impinges upon a smoke particle having a surface parallel to that of the substrate. In addition, since the smoke particles are not in intimate contact with the substrate as in prior art surfaces, incident light which penetrates through the surface to the substrate is subject to further reflection at the substrate, the light thus being reflected back into the smoke layer. As a result, an extremely high probability of total absorption of incident light energy is afforded. v

The penetration depth of incident light is a direct function of its wavelength. Maximum absorption for a given wavelength occurs for a layer which is approximately a quarter wavelength thick. As earlier noted, the near infrared region is of substantial interest today, and the smoke photoemitters of the invention are particularly well suited for use in such systems. The range of wavelengths in this region is from 7,000 A to 15,000 A. To provide a layer having a thickness corresponding to a quarter of such wavelengths requires a photosurface of greater thickness than can be practically realized with prior art, hard vacuum deposited photoemitters. Typically, as illustrated in FIG. 3a, the prior art photoemitter is approximately 50 nanometers in thickness whereas the photoemitter of the invention may be of 1,000 nanometers or more. Whereas the photoemitter of the invention presents little or no reflectance despite that thickness, substantially higher reflectance is presented by the prior art photoemitter despite its being much thinner. As earlier noted, any effort to increase the thickness of a prior art surface to approach that of the photoemitter of the invention would result in a concomitant increase in reflectance and an intolerable decrease in the quantum efficiency.

Numerous other reasons may also be advanced for explanation of the improved operation of the photosurface of the invention. For example, the substantial increase in the photon absorption coefficient of the smoke photoemitter affords a greatly increased probability that the hot electrons can escape from the silver into the cesium oxide for generating free photoelectrons in view of the shorter escape depthsfrom the silver into the cesium oxide. Further, due to the porosity of the low density smoke, the probability of escape of the released photoelectrons is also greatly increased.

As previously noted, the 8-1 or silver-oxygen-cesium surface has heretofore been recognized to afford the highest quantum efficiency of conventional photosurfaces in the near infrared region. The graph of FIG. 4 illustrates the improved response of a photosurface of the 8-1 type produced in accordance with the invention with an S-l photosurface prepared by conventional techniques. Although the response levels approach each other in the visible spectrum, the infrared peak of the smoke photoemitter of the invention is much more distinct than that of the conventional S-l surface.

The technique of smoke deposition for photoemissive surfaces appears to have general applicability, enhancing the quantum efficiency not only of photoemitters having metallic constituents but also of those having solely semiconducting constituents and, as in the case of the 8-1 surface, of those having a combination thereof. Other examples of combined metallicsemiconducting surfaces include silver-oxygenrubidium and silver-bismuth-oxygen-cesium. In each instance, a smoke deposition of the metallic ingredient is substituted for the prior art hard vacuum deposition. This observation is confirmed by the improved characteristics obtained in the formation of 8-20 (alkali antimonide) surfaces, Le, a surface having purely semiconducting constituents.

The processing steps in the formation of the 8-20 surface in accordance with the invention were as follows. As before, the chamber was decontaminated and evacuated. The smoke deposition was achieved by forming a smoke of antimony, again generally at room temperature in the range of 20 to 30C, and with an inert gas of either helium or argon at a pressure of 3 to 4mm Hg in the chamber. The source of antimony is heated to a temperature of approximately 750C for 15 to 60 seconds to develop the smoke deposit. A conventional hard vacuum deposition of the alkali metals was then performed at an operating temperature of about 200C for a period of 60 minutes.

It is apparent that various modifications and variations may be made in the process described herein without departure from the scope of the invention. Fur ther, the specific smoke particle size which is developed will be a function of many variables, including particularly the inert gas pressure, and the particle size will, of course, vary in a given surface. In accordance with the teachings of this invention, a photoemissive material is deposited to form a low densitylayer not greater than 20% and preferably not greater than 5% What is claimed is:

1. A photoemitter responsive to incident photons to generate photoelectrons, comprising:

a. a substrate;

b. a first porous layer of a first constituent of a photoemissive material disposed upon said substrate for reducing reflective and transmissive losses, the degree of porosity being such that the density of said porous layer is less than 20% of the solid state 'density of said first constituent; and

c. a first high density layer of a second constituent of said photoemissive material disposed upon said first porous layer.

2. A photoemitter as claimed in claim 1, wherein said first porous layer comprises a porous layer of particles loosely, linked together and defining voids therebetween. g

3. A photoemitter as claimed in claim 1, wherein the first constituent comprises a metallic substance.

4. A photoemitter as claimed in claim 3, wherein said metallic substance comprises silver.

5. A photoemitter as claimed in claim 4, wherein said second constituent comprises cesium and oxygen,

thereby yielding said first high density 'layer of cesium oxide.

6. A photoemitter as claimed in claim 1, wherein said first constituent comprises a semiconductive compound including at least one element thereof selected from each of Groups Ill and V of the Periodic Table.

yielding said first high density layer of antimonide. 

2. A photoemitter as claimed in claim 1, wherein said first porous layer comprises a porous layer of particles loosely linked together and defining voids therebetween.
 3. A photoemitter as claimed in claim 1, wherein the first constituent comprises a metallic substance.
 4. A photoemitter as claimed in claim 3, wherein said metallic substance comprises silver.
 5. A photoemitter as claimed in claim 4, wherein said second constituent comprises cesium and oxygen, thereby yielding said first high density layer of cesium oxide.
 6. A photoemitter as claimed in claim 1, wherein said first constituent comprises a semiconductive compound including at least one element thereof selected from each of Groups III and V of the Periodic Table.
 7. A photoemitter as claimed in claim 6, wherein said second constituent comprises an alkali metal.
 8. A photoemitter as claimed in claim 1, wherein said first constituent comprises antimony, and said second constituent comprises alkali metals and oxygen thereby yielding said first high density layer of antimonide. 